Powder metallurgy is routinely used to produce a variety of simple- and complex-geometry carbon steel components requiring close dimensional tolerances, good strength and wear resistant properties. The technique involves pressing metal powders that have been premixed with organic lubricants into useful shapes and then sintering them at high temperatures in continuous furnaces into finished products in the presence of controlled atmospheres.
The overall cost of producing components by powder metallurgy has been known to be greatly affected by both the time and money spent on maintaining furnaces and by the cost of controlled atmospheres. The productivity and quality of components, on the other hand, are affected by furnace downtime and consistent composition of controlled atmospheres, respectively. Therefore, there is a need to develop processes and/or atmospheres that will assist in reducing downtime and maintenance costs and improving quality and productivity of components produced by powder metallurgy.
The continuous sintering furnaces normally contain three distinct zones, i.e., a preheating zone, a high heating zone, and a cooling zone. The preheating zone is used to preheat components to a predetermined temperature and to thermally assist in removing organic lubricants from components. The high heating zone is obviously used to sinter components, and the cooling zone is used to cool components prior to discharging them from continuous furnaces.
The high heating zones of continuous furnaces used for sintering steel components are generally operated at temperatures above about 1,000.degree. C. Because of high temperature operation, expensive, high temperature nickel-chromium containing alloys such as Inconel or relatively inexpensive stainless steels are generally used to build sintering furnaces. This is particularly true for building high heating zones of continuous furnaces. The use of these expensive, high temperature alloys helps in prolonging life of continuous furnaces and concomitantly reducing maintenance costs.
The continuous mesh belts used to load and unload components in continuous furnaces are generally made of either expensive, high temperature nickel-chromium containing alloys such as Inconel or relatively inexpensive stainless steels. The expensive, high temperature nickel-chromium containing alloys are preferred materials for building wire mesh belts and obtaining longer life, but they are cost prohibitive and seldom used by the Powder Metal Industry. Although stainless steel mesh belts require frequent maintenance, they are commonly used by the Powder Metal Industry because they are relatively inexpensive.
The controlled atmospheres used for sintering steel components are generally produced and supplied by endothermic generators, ammonia dissociators, or by simply blending pure nitrogen with hydrogen. The endothermic atmospheres are produced by catalytically combusting controlled amount of a hydrocarbon gas, such as natural gas in air in endothermic generators. The endothermic atmospheres typically contain nitrogen (.about.40%), hydrogen (.about.40%), carbon monoxide (.about.20%), and low levels of impurities, such as carbon dioxide, oxygen, and methane. The atmospheres produced by dissociating ammonia contain hydrogen (.about.75%), nitrogen (.about.25%), and impurities in the form of undissociated ammonia, oxygen, and moisture. The composition and level of impurities present in endothermically produced atmospheres and those produced by dissociating ammonia are known to change with time, due to catalyst degradation, continuous changes in composition of the feed stock, or leaks in the system caused by high-temperature operation. The changes in the composition and impurity levels in these atmospheres present problems in providing a decent carbon control and producing parts reproducibly with consistent quality. Also, there is always a threat of exposing workers to environmentally unfriendly and harmful carbon monoxide and ammonia with the use of these endothermically generated and dissociated ammonia atmospheres, respectively. Therefore, the Powder Metal Industry has been moving away from using these endothermically generated and dissociated ammonia atmospheres for sintering steel components requiring good carbon control, consistent quality and properties.
Nitrogen-hydrogen atmospheres produced by blending pure nitrogen with hydrogen have been used by the Powder Metal Industry for more than 15 years as alternatives to endothermically generated and dissociated ammonia atmospheres. Because these atmospheres are produced by blending pure nitrogen and hydrogen, they avoid problems associated with the exposure of workers to environmentally unfriendly and harmful gases. Furthermore, since the composition and flow rates of these atmospheres can be easily changed and precisely controlled, they have been widely accepted by the Powder Metal Industry for sintering steel components that require good carbon control, consistent quality and properties.
Although pure nitrogen-hydrogen atmospheres containing less than 5 ppm oxygen and -62.degree. C. [-80.degree. F.] dew point (less than 10 ppm moisture) have been very useful in producing steel components with good quality, consistency, and properties, they have been found to impact negatively on the life of wire mesh belts made of both expensive, nickel-chromium containing alloys and relatively inexpensive stainless steels, thereby increasing downtime and maintenance costs. Therefore, there is a need to develop improved nitrogen-hydrogen based atmospheres for producing steel components by powder metallurgy with consistent quality and properties while improving life of wire mesh belts and reducing downtime and maintenance costs.